Studies on the fine structure of the frog neuromuscular junction in relation to the role that Ca ions play in depolarization-secretion coupling will be carried out. We will test the hypothesis that decrements in synaptic activity following prolonged or intense stimulation occur not primarily because of depletion of synaptic vesicles or of transmitter, but rather as a result of the transient deactivation of the Ca ions binding capacity of the pre-synaptic ending -- especially of the synaptic vesicles. The localization of calcium will be determined visually and quantitatively with x-ray microanalysis. Synaptogenesis in the rostral anteroventral cochlear nucleus of the postnatal cat will be correlated with emerging function.